US6440703B1 - Enzymatic synthesis of gangliosides - Google Patents

Enzymatic synthesis of gangliosides Download PDF

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US6440703B1
US6440703B1 US09/935,363 US93536301A US6440703B1 US 6440703 B1 US6440703 B1 US 6440703B1 US 93536301 A US93536301 A US 93536301A US 6440703 B1 US6440703 B1 US 6440703B1
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sphingoid
glycosyltransferase
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Shawn DeFrees
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Seneb Biosciences Inc
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    • C12P—FERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE
    • C12P19/00—Preparation of compounds containing saccharide radicals
    • C12P19/26—Preparation of nitrogen-containing carbohydrates
    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12P—FERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE
    • C12P19/00—Preparation of compounds containing saccharide radicals
    • C12P19/18—Preparation of compounds containing saccharide radicals produced by the action of a glycosyl transferase, e.g. alpha-, beta- or gamma-cyclodextrins

Definitions

  • Gangliosides are a class of glycolipids, often found in cell membranes, that consist of three elements. One or more sialic acid residues are attached to an oligosaccharide or carbohydrate core moiety, which in turn is attached to a hydrophobic lipid (ceramide) structure which generally is embedded in the cell membrane.
  • the ceramide moiety includes a long chain base (LCB) portion and a fatty acid (FA) portion.
  • LCB long chain base
  • FA fatty acid
  • Gangliosides are classified according to the number of monosaccharides in the carbohydrate moiety, as well as the number and location of sialic acid groups present in the carbohydrate moiety. Monosialogangliosides are given the designation “GM”, disialogangliosides are designated “GD”, trisialogangliosides “GT”, and tetrasialogangliosides are designated “GQ”. Gangliosides can be classified further depending on the position or positions of the sialic acid residue or residues bound.
  • Gangliosides are most abundant in the brain, particularly in nerve endings. They are believed to be present at receptor sites for neurotransmitters, including acetylcholine, and can also act as specific receptors for other biological macromolecules, including interferon, hormones, viruses, bacterial toxins, and the like. Gangliosides are have been used for treatment of nervous system disorders, including cerebral ischemic strokes. See, e g., Mahadnik et al. (1988) Drug Development Res. 15: 337-360; U.S. Pat. Nos. 4,710,490 and 4,347,244; Horowitz (1988) Adv. Exp. Med. and Biol. 174: 593-600; Karpiatz et al. (1984) Av. Exp. Med. and Biol. 174: 489-497.
  • gangliosides as therapeutic reagents, as well as the study of ganglioside function, would be facilitated by convenient and efficient methods of synthesizing desired gangliosides.
  • a combined enzymatic and chemical approach to synthesis of 3′-nLM 1 and 6′-nLM 1 has been described (Gaudino and Paulson (1994) J. Am. Chem. Soc. 116: 1149-1150).
  • this and other previously available synthetic methods for ganglioside synthesis suffer from low efficiency and other drawbacks.
  • the present invention fulfills this and other needs.
  • the present invention provides methods for in vitro synthesis of glycosphingoids, including gangliosides, and other oligosaccharide-containing compounds.
  • the methods involve the enzymatic transfer of carbohydrates, including sialic acids, to a sphingoid precursor.
  • the methods involve contacting the sphingoid precursor with one or more glycosyltransferases and the corresponding sugar donor moiety for the glycosyltransferases, and other reactants required for glycosyltransferase activity, for a sufficient time and under appropriate conditions to transfer the sugar or sugars from the donor moiety to the sphingoid precursor.
  • the invention provides methods for adding one or more sialic acid residues to a glycosylated ceramide to form a ganglioside.
  • the glycosylated ceramide is contacted with a sialyltransferase and a sialic acid donor moiety and other reactants required for sialyltransferase activity, under conditions such that sialic acid is transferred from the sialic acid donor moiety to the saccharide moiety of the glycosylated ceramide.
  • FIG. 1 shows a schematic diagram of two methods for synthesis of the ganglioside GM 2 by enzymatic synthesis using as the starting material lactosylceramide obtained from bovine buttermilk.
  • FIG. 2 shows a schematic diagram of two methods for synthesizing the ganglioside GD 2 from lactosylceramide obtained from bovine buttermilk.
  • FIG. 3 shows three routes for synthesizing a GM2 ganglioside using a plant glucosylceramide as the starting material.
  • FIG. 4 shows three routes for synthesizing GM2 and other gangliosides starting from a glucosylceramide.
  • GalNAc N-acetylgalactosaminyl
  • GlcNAc N-acetylglucosaminyl
  • a “sphingoid,” as used herein, includes sphingosines, phytosphingosines, sphinganines, ceramides, and the like. Both naturally occurring and synthetically produced compounds are included.
  • a “glycosphingolipid” is a carbohydrate-containing derivative of a sphingoid or ceramide. The carbohydrate residue is attached by a glycosidic linkage to O-1 of the sphingoid.
  • a third sialic acid family member is 2-keto-3-deoxy-nonulosonic acid (KDN) (Nadano et al. (1986) J. Biol. Chem. 261: 11550-11557; Kanamori et al. (1990) J. Biol. Chem. 265: 21811-21819.
  • KDN 2-keto-3-deoxy-nonulosonic acid
  • 9-substituted sialic acids such as a 9-O—C 1 -C 6 acyl-Neu5Ac like 9-O-lactyl-Neu5Ac or 9-O-acetyl-Neu5Ac, 9-deoxy-9-fluoro-Neu5Ac and 9-azido-9-deoxy-Neu5Ac.
  • sialic acid family see, e.g., Varki (1992) Glycobiology 2: 25-40; Sialic Acids: Chemistry, Metabolism and Function, R. Schauer, Ed. (Springer-Verlag, New York (1992).
  • the synthesis and use of sialic acid compounds in a sialylation procedure is described in, for example, international application WO 92/16640, published Oct. 1, 1992.
  • the present invention provides methods for efficient synthesis of glycolipids, including gangliosides, glycosphingoids, and other glycosylated structures.
  • the invention provides methods for synthesizing gangliosides that are useful for study of ganglioside biological function, as well as for therapeutic applications.
  • the methods involve contacting a glycosylated or unglycosylated ceramide or sphingoid with a glycosyltransferase and a sugar donor moiety for a sufficient time and under appropriate reaction conditions to transfer the sugar from the donor moiety to the ceramide or sphingoid.
  • the enzymatic reaction is carried out in the presence of an organic solvent which can increase the efficiency of the reaction.
  • the fatty acid moiety is removed from the ceramide or sphingoid prior to the glycosyltransferase reaction, thus further increasing the effectiveness of the enzymatic transfer.
  • glycosyltransferase(s) used in a given synthesis method of the invention will depend upon the acceptor which is used as the starting material and the desired end product.
  • a method can involve the use of more than one glycosyltransferase, where more than one saccharide is to be added.
  • the multiple glycosyltransferase reactions can be carried out simultaneously or sequentially.
  • a nucleic acid that encodes the enzyme can be cloned and expressed as a recombinant soluble enzyme by methods known to one of ordinary skill in the art.
  • the invention allows one to synthesize a glycosphingolipid starting from a ceramide or other non-glycosylated sphingoid.
  • the initial enzyme involved in synthesis of a glycoceramide from a ceramide precursor is, depending on the desired end product, either a ceramide glucosyltransferase (EC 2.4.1.80, for glucosylceramide) or a ceramide galactosyltransferase (EC 2.4.1.45, for galactosylceramide) (Step 1).
  • a ceramide glucosyltransferase EC 2.4.1.80, for glucosylceramide
  • a ceramide galactosyltransferase EC 2.4.1.45, for galactosylceramide
  • the acceptor used in these reactions can be any of N-acylsphingosine, sphingosine and dihydrosphingosine.
  • Suitable donor nucleotide sugars for the glycosyltransferase include UDP-Glc and CDP-Glc, while the galactosyltransferase typically uses UDP-Gal as a donor.
  • glycosphingoid-linked oligosaccharides can be first “trimmed,” either in whole or in part, to expose either an acceptor for the sialyltransferase or a moiety to which one or more appropriate residues can be added to obtain a suitable acceptor.
  • Enzymes such as glycosyltransferases and endoglycosidases are useful for the attaching and trimming reactions.
  • the second step in the synthesis of gangliosides involves the addition of a second carbohydrate residue, galactose, to the glucosylsphingoid to form the structure Gal ⁇ 1-4Glc-R, wherein R is ceramide (and the product is lactosylceramide) or other sphingoid (Step 2).
  • This reaction is catalyzed by a UDP-Gal: glucosylceramide ⁇ -1,4-galactosyltransferase (EC 2.4.1.38), which is also referred to as lactosylceramide synthase.
  • This enzyme has been characterized from rat (Nomura et al. (1998) J. Biol. Chem.
  • the galactosylation is carried out using a ⁇ -galactosidase.
  • the glycosphingoid precursor is contacted ⁇ -galactosidase and a compound having the formula Gal-X, where X is a leaving group attached to the 1-position of the galactose residue, under conditions suitable for transfer of the Gal residue from the Gal-X to the glucosylceramide.
  • Suitable leaving groups include, for example, ⁇ -p-nibrophenyl, phenyl, methyl, methoxymethyl, and methoxyethyl ethers.
  • Other activating groups are discussed in, for example, Nilsson et al. (1988) Trends Biotechnol.
  • the glycosyltransferase used to add the next saccharide moiety (Step 3) will depend on the particular glycosphingolipid to be synthesized.
  • the invention provides methods of adding one or more sialic acid residues to a glycosylceramide to form a ganglioside. These methods make use of sialyltransferases, which comprise a family of glycosyltransferases that transfer sialic acid from the donor substrate CMP-sialic acid to acceptor oligosaccharide substrates.
  • acceptor for the sialyltransferase will be present on the glycosylceramide to be modified by the methods of the present invention.
  • Suitable acceptors include, for example, galactosyl acceptors such as Gal ⁇ 1,4GalNAc-, Gal ⁇ 1,3GalNAc-, lacto-N-tetraose-, Gal ⁇ 1,3GlcNAc-, Gal ⁇ 1,4GlcNAc-, Gal ⁇ 1,3Ara-, Gal ⁇ 1,6GlcNAc-, and Gal ⁇ 1,4Glc-(lactose).
  • Other acceptors known to those of skill in the art (see, e.g., Paulson et al. (1978) J. Biol. Chem. 253: 5617-5624).
  • the acceptors form part of an oligosaccharide chain that is attached to a ceramide or other glycosphingoid moiety.
  • sialyltransferase employed in the reactions will depend upon the ganglioside being synthesized.
  • GM3 for example, an ⁇ 2,3 sialyltransferase is used, along with its acceptor CMP-sialic acid.
  • a suitable enzyme for this reaction is described in Ishii (1997) Glycoconj. J. 14(supp. 1): S49.
  • Additional gangliosides can be synthesized by contacting ⁇ 2,3-sialylated moieties with an ⁇ 2,8-sialyltransferase, either after or simultaneously with the ⁇ 2,3 sialyltransferase reaction.
  • the ⁇ 2,8-sialyltransferase catalyzes the addition of a sialic acid residue linked ⁇ 2,8 to the ⁇ 2-3-linked sialic acid.
  • this method one can synthesize the gangliosides GD1a, GD1b, GT1a, GT1b, GT1c, and GQ1b, for example.
  • the structures for these gangliosides, as well as those discussed above, are shown in Table 2.
  • an ⁇ 2,8 sialyltransferase is used, either simultaneously with or after the reaction with a ⁇ 2,3 sialyltransferase.
  • a suitable enzyme for this reaction is the ST8Sia I (GD3/GT3 synthase; EC 2.4.99.8; see, e.g., (human) Nara et al. (1994) Proc. Natl. Acad. Sci USA 91: 7952-7956; Haraguchi et al. (1994) Proc. Natl. Acad. Sci. USA 91:10455-10459; Sasaki et al. (1994) J. Biol. Chem.
  • ⁇ 2-8-sialylatransferases include ST8SiaII, STX (Scheidegger et al. (1995) J. Biol. Chem. 270: 22685-22688) and ST8SiaIII, which utilizes Sia2,3Gal1,4GlcNAc as an acceptor, and thus is useful in methods for synthesizing neolactogangliosides (Yoshida et al. (1995) J. Biol. Chem. 270: 14628-14633).
  • the two sialyltransferase reactions are carried out simultaneously.
  • sialyltransferase For sialylation of the lacto- and neolacto-series of gangliosides, the sialyltransferase will be able to transfer sialic acid to the structures Gal ⁇ 1,4GlcNAc- and Gal ⁇ 1,4GlcNAc-, respectively.
  • Suitable sialyltransferases include those that are summarized in Table 1.
  • Sialyltransferases which use the Gal ⁇ 1,4GlcNAc sequence as an acceptor substrate.
  • Sialyltransferase Source Sequence(s) formed Ref.
  • ST6Gal I Mammalian NeuAc ⁇ 2,6Gal ⁇ 1,4GlcNAc- 1 ST3Gal III
  • the lactosylceramide can also be utilized as the acceptor for a ⁇ 1,4-galactosaminyltransferase (EC 2.4.1.92), which catalyzes the transfer of a GalNAc from UDP-GalNAc to the Gal moiety of the lactosylceramide.
  • EC 2.4.1.92 ⁇ 1,4-galactosaminyltransferase
  • the nucleotide sequence for this enzyme is available for human (GenBank Accession No. M83651; Nagata et al. (1992) J. Biol. Chem. 267: 12082-12087), rat (GenBank Accession No. D17809;Hidari et al. (1994) Biochem. J. 301: 957-965), and mouse (GenBank Accession No. L25885; Sango et al. (1995) Genomics 27: 362-365), for example. Therefore, the enzyme is readily obtainable by recombinant methods.
  • an ⁇ 2,3 sialyltransferase is employed, as discussed above.
  • the ganglioside GD2 can be synthesized by the methods of the invention by use of an ⁇ 2,8 sialyltransferase following, or simultaneously with, the reaction with the ⁇ 2,3 sialyltransferase.
  • an ⁇ 2,6 sialyltransferase ST6GalNAcI; EC 2.4.99.3 can be employed to add a sialic acid residue in an ⁇ 2,6 linkage (Kurosawa et al. (1994) J. Biol. Chem. 269: 1402-1409).
  • the GalNAc ⁇ 4Gal ⁇ 4Glc-Cer moiety can also be used as the acceptor for synthesis of additional glycosphingolipids.
  • the invention provides methods in which this compound is galactosylated using a ⁇ 1,3 galactosyltransferase.
  • a suitable galactosyltransferase for this application is described in, for example, Ghosh et al. (1995) Glycoconj. J. 12: 838-47.
  • the methods of the invention involve contacting the moiety with an ⁇ 2,3 sialyltransferase.
  • the methods can use a ST3Gal III sialyltransferase (human: Sasaki et al. (1993) J. Biol. Chem. 268: 22782-22787; GenBank Accession No. x74570).
  • ST3Gal III sialyltransferase human: Sasaki et al. (1993) J. Biol. Chem. 268: 22782-22787; GenBank Accession No. x74570.
  • the bacterial sialyltransferases are also suitable for this reaction (Table 1).
  • the invention also provides methods of adding sialic acid in an ⁇ 2,6Gal linkage.
  • ST6Gal I can catalyze the addition of a sialic acid in an ⁇ 2,6Gal linkage.
  • This enzyme is commercially available (Boehringer Mannheim Biochemicals, Indianapolis Ind.), and the cDNA has been cloned from several organisms, including rat (Weinstein et al. (1987) J. Biol Chem. 262: 17735-17743), human (Grundmann et al. (1990) Nucl. Acids Res. 18:667; Zettlmeisl et al. (1992) Patent EP O475354; Stamenkovic et al. (1990) J.
  • gangliosides can be synthesized using ST6GalNAcII, which adds an ⁇ 2,6-linked sialic acid to the Gal ⁇ 3GalNAc-moiety (chicken, Kurosawa et al. (1994) J. Biol. Chem. 269: 19048-19053, GenBank Accession No. x77775).
  • FIG. 2 shows a schematic diagram of two pathways for synthesis of the ganglioside GD 2 starting from lactosylceramide. Each pathway involves the use of two different sialyltransferases (an ⁇ 2,3ST and an ⁇ 2,8ST), as well as a GalNAc transferase. In the preferred pathway, the fatty acid is removed from the lactosylceramide by treatment with base (Step 1).
  • Acetylation is then performed (Step 2), after which a sialic acid is attached to the galactose residue in an ⁇ 2,3 linkage by an ⁇ 2,3 sialyltransferase (Step 3).
  • the sialylation steps are performed, preferably in the presence of an organic solvent as described herein, thereby driving the reaction nearly to completion.
  • a GalNAc residue is then added to the galactose in a ⁇ 1,4 linkage using a GalNAc transferase (Step 5).
  • a fatty acid is added, e.g., by reaction with steroyl chloride, to complete the ganglioside (Step 6).
  • glycosyltransferases alone or in combination, that can catalyze the synthesis of other glycosphingolipids of interest. For example, to synthesize a lacto- or neolacto-glycosphingolipid, one would replace the ⁇ 1,4 galactosaminyltransferase in Step 3 above with a ⁇ 1,3GlcNAc transferase.
  • a ⁇ 1,3 galactosyltransferase (lacto) or a ⁇ 1,4 galactosyltransferase (neolacto) will then provide the acceptor for the appropriate sialyltransferase or sialyltransferases to synthesize a ganglioside, as desired.
  • An example of a sialyltransferase that is useful in these methods is ST3Gal III, which is also referred to as ⁇ (2,3)sialyltransferase (EC 2.4.99.6).
  • This enzyme catalyzes the transfer of sialic acid to the Gal of a Gal ⁇ 1,3GlcNAc or Gal ⁇ 1,4GlcNAc glycoside (see, e.g., Wen et al. (1992) J. Biol. Chem., 267: 21011-21019; Van den Eijnden et al. (1991) J. Biol. Chem., 256: 3159).
  • the sialic acid is linked to a Gal with the formation of an ⁇ -linkage between the two saccharides. Bonding (linkage) between the saccharides is between the 2-position of NeuAc and the 3-position of Gal.
  • This particular enzyme can be isolated from rat liver (Weinstein et al. (1982) J. Biol.
  • the products of these reactions are subjected to ⁇ 1,3GalNAc transferase-catalyzed addition of a GalNAc residue to the nonreducing Gal residue.
  • treatment of the product with one or more sialyltransferases will yield a desired ganglioside.
  • Gangliosides and other glycosphingolipids sometimes include other sugars in addition to those described above.
  • fucose residues are sometimes present.
  • the invention provides methods of synthesizing these fucosylated glycosphingolipids. These methods involve the use of a fucosyltransferase to catalyze the transfer of a fucose residue from an activated nucleotide sugar (GDP-fucose) to an appropriate acceptor.
  • GDP-fucose activated nucleotide sugar
  • a GM1a ganglioside with an ⁇ 1,2-fucosyltransferase to obtain a ganglioside having the structure Fuc ⁇ 2Gal ⁇ 3GalNAc ⁇ 4 (Sia ⁇ 3)Gal ⁇ 4Glc-Cer (Wiegandt (1973) H.-S. Zschr. Physiol. Chem. 354: 1049-1056).
  • Gangliosides of interest are described in Oettgen, H. F., ed., Gangliosides and Cancer, VCH, Germany, 1989, pp. 10-15, and references cited therein.
  • Gangliosides of particular interest include, for example, those found in the brain as well as other sources which are listed in Table 2.
  • the invention provides methods for in vitro sialylation of saccharide groups present on a glycosylceramide, wherein the methods first involve modifying the glycosylceramide to create a suitable acceptor.
  • a preferred method for synthesizing an acceptor involves use of a galactosyltransferase. The steps for these methods include:
  • this product is contacted with an ⁇ 2,8-sialyltransferase under conditions in which a sialic acid residue is transferred to the ⁇ 2,3-linked sialic acid to form the compound NeuAc ⁇ (2-8)NeuAc ⁇ (2-3)Gal ⁇ (1-4)Glc- ⁇ -OR.
  • the Gal ⁇ (1-4)Glc ⁇ -OR compound formed in step (a) is further modified, either before or after the sialylation step (b).
  • the methods can involve the additional steps of:
  • step (d) adding a Gal residue to GalNAc ⁇ (1-4) Gal ⁇ (1-4)Glc ⁇ -OR by contacting the compound formed in step (c) with a ⁇ 1-3-galactosyltransferase in the presence of UDP-Gal, under conditions in which Gal is transferred to the non-reducing end of the oligosaccharide to form Gal ⁇ (1-3)GalNAc ⁇ (1-4)Gal ⁇ (1-4)Glc ⁇ -OR.
  • step (e) contacting the product of step (d) with one or more sialyltransferases as described in step (b), under conditions in which sialic acid is transferred to either or both of the Gal residues.
  • This reaction can involve, for example, an ⁇ 2-3-sialyltransferase alone, or an ⁇ 2-3-sialyltransferase and an ⁇ 2,8-sialyltransferase.
  • the sphingoids that can be used as starting materials in the methods of the invention include, but are not limited to, those that have the formula I:
  • R 1 is selected from the group consisting of H, Glc ⁇ 1-, Gal ⁇ 1-, and lactose ⁇ 1-;
  • R 2 is selected from the group consisting of CH 2 and
  • R 3 is selected from the group consisting of H, CH 3 , CH 2 CH 3 ,
  • R 4 is selected from the group consisting of:
  • R 6 is either a divalent C 2 -C 36 alkyl or a divalent ⁇ -hydroxy-C 2 -C 36 alkyl
  • R 7 is either a monovalent C 2 -C 36 alkyl or a monovalent ⁇ -hydroxy-C 2 -C 36 alkyl
  • R 5 is selected from the group consisting of a saturated, unsaturated, or polyunsaturated C 2 -C 37 alkyl group.
  • the sphingoids comprise the formula II:
  • R 1 is selected from the group consisting of H, Glc ⁇ 1-, Gal ⁇ 1-, lactose ⁇ 1-, and an oligosaccharide;
  • R 2 is selected from the group consisting of H, a saturated or unsaturated C 2 -C 26 alkyl group, and a protecting group;
  • R 3 is a saturated, unsaturated, or polyunsaturated C 2 -C 37 alkyl group, or a protecting group.
  • Sphingoids of particular interest include, for example, sphingosines, phytosphingosines, sphinganines, and ceramides.
  • the sphingoids can be naturally occurring or can be produced synthetically or semisynthetically.
  • An example of a semisynthetically produced ganglioside derivative that one can produce using the methods of the invention is the N-dichloroacetylsphingosine compounds described in Kharlamov et al. (1994) Proc. Nat'l. Acad. Sci. USA 91: 6303-6307 (e.g., LIGA20) and Schneider et al.
  • R 2 in the above formula II includes the lipid moiety 2-dichloroacetylamide.
  • the dicloroacetylsphingosine compounds have R 3 in formula II as being 4-trans-octadecene.
  • the invention provides methods of synthesizing glycosphingoids having the formula III:
  • X 1 is a sphingoid, such as a ceramide or a sphingosine, for example;
  • X 2 and X 4 are each independently selected from the group consisting of -H, Sia ⁇ 2-3-, Sia ⁇ 2-6-, Sia ⁇ 2-8-Sia ⁇ 2-3-, and Fuc ⁇ 1-2-;
  • X 3 is optional and, if present, is selected from the group consisting of GalNAc ⁇ 1-4-, Gal ⁇ 1-3GalNAc ⁇ 1-4-, Fuc ⁇ 1-2Gal ⁇ 1-3GalNAc ⁇ 1-4-, Gal ⁇ 1-3GlcNAc ⁇ 1-3-, Gal ⁇ 1-4GlcNAc ⁇ 1-3-, GalNAc ⁇ 1-3Gal ⁇ 1-4-, and GalNAc ⁇ 1-3Gal ⁇ 1-3-.
  • the present invention also provides methods of synthesizing analogs of gangliosides and other glycosphingolipids.
  • the analog is chosen so that the analog-nucleotide is still capable of serving as a donor for the glycosyltransferase of interest.
  • suitable analogs for synthesizing gangliosides are described in, for example, U.S. Pat. No. 5,352,670.
  • the methods of the invention involve removal of a fatty acid moiety from a glycoceramide or sphingoid prior to reaction with the glycosyltransferase.
  • Methods of removing a fatty acid moiety from a glycosphingolipid are known to those of skill in the art. Standard carbohydrate and glycosphingolipid chemistry methodology can be employed, such as that described in, for example, Paulson et al. (1985) Carbohydrate Res. 137: 39-62; Beith-Halahmi et al. (1967) Carbohydrate Res. 5: 25-30; Alais and Veyrieries (1990) Carbohydrate Res.
  • the glycosyltransferase reactions are carried out in the presence of an organic solvent.
  • the fatty acid moiety is first hydrolyzed as described above.
  • the enzymatic catalyses can be carried out in the presence of an organic solvent, such as, for example, methanol, ethanol, dimethylsulfoxide, isopropanol, tetrahydrofuran, chloroform, and the like, either singly or in combination.
  • the proportion of the organic solvent in the reaction mixture is typically at least about 3%, more preferably at least about 5%, and most preferably at least about 8%.
  • the reaction mixture typically contains about 25% or less of organic solvent, more preferably about 20% or less, and most preferably about 10% or less organic solvent. In a presently preferred embodiment, the reaction mixture contains about 8-10% methanol.
  • an organic solvent as provided by the present invention not only results in a much higher yield than was obtainable previously, it also eliminates the need for a detergent to increase accessibility to the glycosyl moiety of the glycosylceramide. This facilitates purification of the resulting ganglioside.
  • detergents can also be used in the methods of the invention.
  • the glycosylation steps in the methods of the invention are preferably carried out enzymatically.
  • a plurality of enzymatic steps are carried out in a single reaction mixture that contains two or more different glycosyltransferases.
  • the enzymes and substrates can be combined in an initial reaction mixture, or preferably the enzymes and reagents for a second glycosyltransferase cycle can be added to the reaction medium once the first glycosyltransferase cycle has neared completion.
  • Enzyme amounts or concentrations are expressed in activity Units, which is a measure of the initial rate of catalysis.
  • One activity Unit catalyzes the formation of 1 ⁇ mol of product per minute at a given temperature (typically 37° C.) and pH value (typically 7.5).
  • 10 Units of an enzyme is a catalytic amount of that enzyme where 10 ⁇ mols of substrate are converted to 10 ⁇ mol of product in one minute at a temperature of 37° C. and a pH value of 7.5.
  • the enzymes can be utilized free in solution or can be bound to a support such as a polymer.
  • the reaction mixture is thus substantially homogeneous at the beginning, although some precipitate can form during the reaction.
  • the glycosylation reactions include, in addition to the appropriate glycosyltransferase and acceptor, an activated nucleotide sugar that acts as a sugar donor for the glycosyltransferase.
  • the reactions can also include other ingredients that facilitate glycosyltransferase activity. These ingredients can include a divalent cation (e.g., Mg +2 or Mn +2 ), materials necessary for ATP regeneration, phosphate ions, and organic solvents.
  • concentrations or amounts of the various reactants used in the processes depend upon numerous factors including reaction conditions such as temperature and pH value, and the choice and amount of acceptor saccharides to be glycosylated.
  • the reaction medium may also comprise solubilizing detergents (e.g., Triton or SDS) and organic solvents such as methanol or ethanol, if necessary.
  • the above ingredients can be combined by admixture in an aqueous reaction medium (solution) which has a pH value of about 6 to about 8.5.
  • the medium is devoid of chelators that bind enzyme cofactors such as Mg +2 or Mn +2 .
  • the selection of a medium is based on the ability of the medium to maintain pH value at the desired level.
  • the medium is buffered to a pH value of about 7.5, preferably with HEPES. If a buffer is not used, the pH of the medium should be maintained at about 6 to 8.5, preferably about 7.2 to 7.8, by the addition of base.
  • a suitable base is NaOH, preferably 6 M NaOH.
  • the temperature at which an above process is carried out can range from just above freezing to the temperature at which the most sensitive enzyme denatures. That temperature range is preferably about zero degrees C. to about 110° C., and more preferably at about 20° C. to about 30° C., or higher for a thermophilic organism.
  • the reaction mixture so formed is maintained for a period of time sufficient for the glycosyltransferase(s) to glycosylate a high percentage of the acceptors. Some of the product can often be detected after a few hours, with recoverable amounts usually being obtained within 24 hours. For commercial-scale preparations, the reaction will often be allowed to proceed for about 8-240 hours, with a time of between about 24 and 48 hours more typical.
  • the glycosylation steps are carried out using a half-cycle or full cycle in which one or more reaction components are regenerated.
  • Glycosyltransferase cycles and half-cycles are described in U.S. Pat. No. 5,728,554.
  • the galactosylating step can be carried out as part of a galactosyltransferase cycle and the sialylating step is preferably carried out as part of a sialyltransferase cycle.
  • Preferred conditions and descriptions of other species and enzymes in each of these, and other, cycles have been described. See, e.g., commonly assigned U.S. Provisional Application No. 60/071,076, filed Jan. 15, 1998 and U.S. patent application Ser. No. 08/628,543, filed Apr. 10, 1996.
  • the sialylation of the glycosylceramide can be accomplished using a sialyltransferase cycle, which includes a CMP-sialic acid recycling system that utilizes CMP-sialic acid synthetase.
  • CMP-sialic acid is relatively expensive, so in situ synthesis of this sialic acid donor moiety enhances the economic advantages provided by the claimed methods.
  • Sialyltransferase cycles are described, for example, in U.S. Pat. No. 5,374,541.
  • the CMP-sialic acid regenerating system used in this embodiment comprises cytidine monophosphate (CMP), a nucleoside triphosphate, a phosphate donor, a kinase capable of transferring phosphate from the phosphate donor to nucleoside diphosphates and a nucleoside monophosphate kinase capable of transferring the terminal phosphate from a nucleoside triphosphate to CMP.
  • CMP cytidine monophosphate
  • CMP cytidine monophosphate
  • nucleoside triphosphate a nucleoside triphosphate
  • a phosphate donor a kinase capable of transferring phosphate from the phosphate donor to nucleoside diphosphates
  • a nucleoside monophosphate kinase capable of transferring the terminal phosphate from a nucleoside triphosphate to CMP.
  • the regenerating system also employs CMP-sialic acid synthetase, which transfers sialic acid to CMP.
  • CMP-sialic acid synthetase can be isolated and purified from cells and tissues containing the synthetase enzyme by procedures well known in the art. See, for example, Gross et al. (1987) Eur. J. Biochem., 168: 595; Vijay et al. (1975) J. Biol. Chem. 250: 164; Zapata et al. (1989) J. Biol. Chem. 264: 14769; and Higa et al. (1985) J. Biol. Chem. 260: 8838. The gene for this enzyme has also been sequenced.
  • Nucleoside triphosphates suitable for use in accordance with the CMP-sialic acid regenerating system are adenosine triphosphate (ATP), cytidine triphosphate (CTP), uridine triphosphate (UTP), guanosine triphosphate (GTP), inosine triphosphate (ITP) and thymidine triphosphate (TTP).
  • ATP adenosine triphosphate
  • CTP cytidine triphosphate
  • UTP uridine triphosphate
  • GTP guanosine triphosphate
  • ITP inosine triphosphate
  • TTP thymidine triphosphate
  • a preferred nucleoside triphosphate is ATP.
  • Nucleoside monophosphate kinases are enzymes that catalyze the phosphorylation of nucleoside monophosphates.
  • Nucleoside monophosphate kinase (NMK) or myokinase (MK; EC 2.7.4.3) used in accordance with the CMP-sialic acid regenerating system of the present invention are used to catalyze the phosphorylation of CMP.
  • NMK's are commercially available (Sigma Chem. Co., St. Louis, Mo.; Boehringer Mannheim, Indianapolis, Ind.).
  • a phosphate donor and a catalytic amount of a kinase that catalyzes the transfer of phosphate from the phosphate donor to an activating nucleotide are also part of the CMP-sialic acid regenerating system.
  • the phosphate donor of the regenerating system is a phosphorylated compound, the phosphate group of which can be used to phosphorylate the nucleoside phosphate.
  • the only limitation on the selection of a phosphate donor is that neither the phosphorylated nor the dephosphorylated forms of the phosphate donor can substantially interfere with any of the reactions involved in the formation of the sialylated galactosyl glycoside.
  • Preferred phosphate donors are phosphoenolpyruvate (PEP) and acetyl phosphate.
  • a particularly preferred phosphate donor is PEP.
  • kinase for use in a sialic acid cycle depends upon the phosphate donor employed.
  • the kinase is acetyl kinase.
  • PEP pyruvate kinase
  • Other kinases can be employed with other phosphate donors as is well known to those of skill in the art. Kinases are commercially available (Sigma Chem. Co., St. Louis, Mo.; Boehringer Mannheim, Indianapolis, Ind.).
  • CMP is converted to CDP by nucleoside monophosphate kinase in the presence of added ATP.
  • ATP is catalytically regenerated from its byproduct, ADP, by pyruvate kinase (PK) in the presence of added phosphoenolpyruvate (PEP).
  • PK pyruvate kinase
  • PEP phosphoenolpyruvate
  • CDP is further converted to CTP, which conversion is catalyzed by PK in the presence of PEP.
  • CTP reacts with sialic acid to form inorganic pyrophosphate (PPi) and CMP-sialic acid, the latter reaction being catalyzed by CMP-sialic acid synthetase.
  • the released CMP re-enters the regenerating system to reform CDP, CTP and CMP-sialic acid.
  • the formed PPi is scavenged as discussed below, and forms inorganic phosphate (Pi) as a byproduct. Pyruvate is also a byproduct.
  • the byproduct pyruvate can also be made use of in another reaction in which N-acetylmannosamine (ManNAc) and pyruvate are reacted in the presence of NeuAc aldolase (EC 4.1.3.3) to form sialic acid.
  • the sialic acid can be replaced by ManNAc and a catalytic amount of NeuAc aldolase.
  • NeuAc aldolase also catalyzes the reverse reaction (NeuAc to ManNAc and pyruvate)
  • the produced NeuAc is irreversibly incorporated into the reaction cycle via CMP-NeuAc catalyzed by CMP-sialic acid synthetase coupled with inorganic pyrophosphatase (PPase)-catalyzed decomposition of the released inorganic pyrophosphate.
  • PPase inorganic pyrophosphatase
  • pyrophosphate scavenger refers to substances that serve to remove inorganic pyrophosphate from a reaction mixture of the present invention.
  • Inorganic pyrophosphate (PPi) is a byproduct of the preparation of CMP-Neu5Ac. Produced PPi can feed back to inhibit other enzymes such that glycosylation is reduced.
  • PPi can be degraded enzymatically or by physical means such as sequestration by a PPi binding substance.
  • PPi is removed by hydrolysis using inorganic pyrophosphatase (PPase; EC 3.6.1.1), a commercially available PPi catabolic enzyme (Sigma Chem. Co., St.
  • a similar enzyme serves as the pyrophosphate scavenger.
  • One method of removing PPi or Pi from the reaction mixture is to maintain divalent metal cation concentration in the medium.
  • the cations and the inorganic phosphate produced form a complex of very low solubility.
  • the rate of reaction can be maintained and the reactions can be taken to completion (ie., 100% conversion).
  • Supplementing can be carried out continuously (e.g., by automation) or discontinuously. When cation concentration is maintained in this way, the transferase reaction cycle can be driven to completion.
  • the concentrations or amounts of the various reactants used in the processes depend upon numerous factors including reaction conditions such as temperature and pH value, and the choice and amount of acceptor saccharides to be glycosylated. Because the glycosylation process permits regeneration of activating nucleotides, activated donor sugars and scavenging of produced PPi in the presence of catalytic amounts of the enzymes, the process is limited by the concentrations or amounts of the stoichiometric substrates discussed before. The upper limit for the concentrations of reactants that can be used in accordance with the method of the present invention is determined by the solubility of such reactants. Preferably, the concentrations of activating nucleotides, phosphate donor, the donor sugar and enzymes are selected such that glycosylation proceeds until the acceptor is consumed, thus completely sialylating the saccharide groups present on the glycoprotein.
  • the glycosylation reactions are conducted by contacting the acceptor with a cell that contains: a) an enzymatic system for producing a nucleotide sugar, and b) a recombinant glycosyltransferase which catalyzes the transfer of a sugar from the nucleotide sugar to the acceptor to produce the desired glycosphingoid.
  • the cells are generally permeabilized and added to the reaction mixture. For reactions that require multiple glycosyltransferase steps, one can use cells that contain more than one recombinant glycosyltransferase and produce the corresponding nucleotide sugar for both glycosyltransferases.
  • glycolipids and gangliosides can be used without purification. However, it is usually preferred to recover the product.
  • standard methods for glycolipid preparation can be used (see, e.g., Ledeen et al. (1973) J. Neurochem. 21:829).
  • glycolipids can be extracted from a reaction mixture by chloroform/methanol 2:1 and isopropyl alcohol/hexane/water 55:25:20 as described by Kannagi et al. (1982) J. Biol. Chem. 257: 14865. The resulting extracts are partitioned by a chloroform/methanol/water (3:2:1) Folch partition.
  • the resulting upper phase of the extraction contains gangliosides and the lower phase contains glycolipids.
  • the upper phase containing gangliosides (glycosphingolipids that contain at least one sialic acid moiety) are isolated and separated into neutral and acidic fractions using DEAE-Sephadex chromatography as described in detail by Ledeen and Yu, Methods Enzymol. 83: 139 (1982).
  • the resulting gangliosides are pooled, lyophilized, and dissolved in chloroform/methanol (2:1).
  • the lower phase of the Folch partition contains glycolipids. These are isolated and separated on preparative thin-layer chromatography using chloroform/methanol/water (60:35:8) as the solvent system as described by Symington.
  • a preferred method of purification involves membrane filtration, more preferably utilizing a reverse osmotic membrane, or one or more column chromatographic techniques for the recovery as is discussed hereinafter and in the literature cited herein.
  • membrane filtration wherein the membranes have molecular weight cutoff of about 3000 to about 10,000 can be used to remove proteins.
  • Nanofiltration or reverse osmosis can then be used to remove salts and/or purify the soluble oligosaccharide products (see, e.g., U.S. patent application Ser. No. 08/947,775, filed Oct. 9, 1997).
  • Nanofilter membranes are a class of reverse osmosis membranes which pass monovalent salts but retain polyvalent salts and uncharged solutes larger than about 100 to about 700 Daltons, depending upon the membrane used. Thus, in a typical application, saccharides prepared by the methods of the present invention will be retained in the membrane and contaminating salts will pass through.
  • immunochemical glycolipid analysis can be performed according to the procedure of Magnani et al. (1980) Anal. Biochem. 109: 399. Briefly, the ganglioside pool described above is chromatographed by thin layer chromatography. The thin layer plate is then incubated with 125 I labeled antibody that binds specifically to the oligosaccharide of interest (e.g., FH6, which binds specifically to SLX). Following incubation with the labeled antibody, the plate is exposed to radiographic detection film and developed.
  • 125 I labeled antibody that binds specifically to the oligosaccharide of interest (e.g., FH6, which binds specifically to SLX).
  • Black spots on the X-ray film correspond to gangliosides that bind to the monoclonal antibody, and those gangliosides are recovered by scraping the corresponding areas of the silica plate and eluting the gangliosides with chloroform/methanol/water. Glycolipids are also dried and resuspended in chloroform and developed in a similar thin layer system and probed with the radiolabeled antibody.
  • the carbohydrate units can be released from the glycosphingoids or glycosylceramides by alkaline ⁇ -elimination, for example, and separated from the ceramide or sphingoid moieties by gel filtration.
  • the resulting oligosaccharides are then separated from each other using a combination of gel filtration, HPLC, thin layer chromatography, and ion exchange chromatography, and can be fully analyzed.
  • Additional techniques to fully characterize the sugars of an oligosaccharide include FAB-MS (fast atom bombardment-mass spectrometry), HPAE (high pH anion exchange chromatography) and 1 H-NMR. These techniques are complementary. Recent examples of how these techniques are used to fully characterize the structure of an oligosaccharide can be found in the analysis by Spellman et al., (1989) J. Biol. Chem. 264: 14100, and Stanley et al. (1988) J. Biol. Chem. 263: 11374.
  • FAB-MS positive ion fast atom bombardment mass spectroscopy
  • GC/EI-MS methylation analysis by gas chromatography-electron impact mass spectroscopy
  • gangliosides other compounds that are made using the methods of the invention can be used in a variety of applications, e.g., as antigens, diagnostic reagents, or as therapeutics.
  • gangliosides have been reported to be useful for treating spinal cord and other nervous system injuries (see, e.g., Skaper et al. (1989) Mol. Neurobiol. 3: 173; Samson (1990) Drug Devel. Rev. 19: 209-224), stroke, subarachnoid hemorrhage, cognition defects (Kharlamov et al. (1994) Proc. Nat'l. Acad. Sci. USA 91: 6303-6307), Parkinson's disease (Schneider (1998) Ann.
  • gangliosides 74: 606-619
  • agents that block or disrupt these gangliosides are useful in reducing the inaccessibility of tumors to the immune system.
  • the immunosuppressive effect of gangliosides is useful for, e.g., suppressing rejection of transplanted organs.
  • the present invention also provides pharmaceutical compositions which can be used in treating a variety of conditions.
  • the pharmaceutical compositions include the gangliosides or glycosphingoids synthesized using the methods of the invention, along with a pharmaceutically acceptable carrier.
  • Pharmaceutical compositions of the invention are suitable for use in a variety of drug delivery systems. Suitable formulations for use in the present invention are found in Remington's Pharmaceutical Sciences, Mace Publishing Company, Philadelphia, Pa., 17th ed. (1985). For a brief review of methods for drug delivery, see, e.g., Langer, Science 249:1527-1533 (1990).
  • compositions are intended for parenteral, intranasal, topical, oral or local administration, such as by aerosol or transdermally, for prophylactic and/or therapeutic treatment.
  • the pharmaceutical compositions are administered parenterally, e.g., intravenously.
  • the invention provides compositions for parenteral administration which comprise the compound dissolved or suspended in an acceptable carrier, preferably an aqueous carrier, e.g., water, buffered water, saline, PBS and the like.
  • an acceptable carrier preferably an aqueous carrier, e.g., water, buffered water, saline, PBS and the like.
  • the compositions may contain pharmaceutically acceptable auxiliary substances as required to approximate physiological conditions, such as pH adjusting and buffering agents, tonicity adjusting agents, wetting agents, detergents and the like.
  • compositions may be sterilized by conventional sterilization techniques, or may be sterile filtered.
  • the resulting aqueous solutions may be packaged for use as is, or lyophilized, the lyophilized preparation being combined with a sterile aqueous carrier prior to administration.
  • the pH of the preparations typically will be between 3 and 11, more preferably from 5 to 9 and most preferably from 7 and 8.
  • the gangliosides and other glycosphingoids made using the invention can be incorporated into liposomes formed from standard vesicle-forming lipids.
  • a variety of methods are available for preparing liposomes, as described in, e.g., Szoka et al., Ann. Rev. Biophys. Bioeng. 9:467 (1980), U.S. Pat. Nos. 4,235,871, 4,501,728 and 4,837,028.
  • the targeting of liposomes using a variety of targeting agents e.g., the oligosaccharide moieties of the gangliosides of the invention
  • U.S. Pat. Nos. 4,957,773 and 4,603,044 see, e.g., U.S. Pat. Nos. 4,957,773 and 4,603,044.
  • compositions containing the gangliosides and other glycosphingoids can be administered for prophylactic and/or therapeutic treatments.
  • compositions are administered to a patient already suffering from a disease, as described above, in an amount sufficient to cure or at least partially arrest the symptoms of the disease and its complications.
  • An amount adequate to accomplish this is defined as a “therapeutically effective dose.” Amounts effective for this use will depend on the severity of the disease and the weight and general state of the patient, but generally range from about 0.5 mg to about 40 g of oligosaccharide per day for a 70 kg patient, with dosages of from about 5 mg to about 20 g of the compounds per day being more commonly used.
  • the gangliosides and other glycosphingoids may also find use as diagnostic reagents.
  • Diagnostic reagents that contain gangliosides made by the methods of the invention, or moieties that bind to the specific gangliosides (e.g., lectins and antibodies), are useful in diagnosing several conditions, including, for example, Fabry disease (-Gal--Gal--GalCer), Farber disease (ceramides; N-acylsphingosines), Gaucher disease (glucocerebroside), GM1 gangliosidosis (GM1 ganglioside), metachromatic leukodystrophy (sulfatide; cerebroside sulfate), Sandhoff disease (GM2 ganglioside), Tay-Sachs disease (GM2 ganglioside).
  • the compounds can be labeled with appropriate labels, including radioisotopes such as, for example, 125 I, 14 C, or tritium.
  • the gangliosides and other glycosphingoids made using the methods of the invention can be used as an immunogen for the production of monoclonal or polyclonal antibodies specifically reactive with the compounds.
  • the multitude of techniques available to those skilled in the art for production and manipulation of various immunoglobulin molecules can be used in the present invention.
  • Antibodies may be produced by a variety of means well known to those of skill in the art. If desired, the production of antibodies can be enhanced by coupling the ganglioside or other glycosphingolipid to an immunogenic protein (e.g.,KLH) prior to administering the compound to the test animal (see, PCT application PCT/US94/00757, Publ. No. WO 94/16731).
  • an immunogenic protein e.g.,KLH
  • Uses for antibodies against gangliosides and other glycosphingolipids include cancer diagnosis and are described in, for example, U.S. Pat. No. 4,887,931.
  • non-human monoclonal antibodies e.g., murine, lagomorpha, equine, etc.
  • production of non-human monoclonal antibodies is well known and may be accomplished by, for example, immunizing the animal with a preparation containing the oligosaccharide of the invention.
  • Antibody-producing cells obtained from the immunized animals are immortalized and screened, or screened first for the production of the desired antibody and then immortalized.
  • Harlow and Lane Antibodies, A Laboratory Manual Cold Spring Harbor Publications, N.Y. (1988).
  • This Example describes the reaction conditions for sialylation of lyso-lactosyl ceramide.
  • Lactosylceramide was obtained from bovine buttermilk and the fatty acid moiety removed by base hydrolysis to form lyso-lactosyl ceramide.
  • ⁇ 2,3 sialyltransferase (10 ⁇ L, 5 U/mL, 50 mU) was then added followed by alkaline phosphatase (1 ⁇ L, 1.0 ⁇ 10 5 U/mL, 100 U).
  • the reaction mixture was kept at room temperature. After one day, a further portion of ⁇ 2,3 sialyltransferase (10 ⁇ L, 5 U/mnL, 50 mU) was added. After four more days, an additional portion of ⁇ 2,3 sialyltransferase (10 ⁇ L, 5 U/mL, 50 mU) was added. After an additional one day at room temperature, thin layer chromatography indicated that the reaction was nearly complete.
  • FIG. 1 A schematic diagram of showing two pathways for synthesis of the ganglioside GM 2 from lactosylceramide obtained from bovine buttermilk is shown in FIG. 1 .
  • the fatty acid is not removed from the lactosylceramide prior to sialylation, and the reaction is not carried out in the presence of an organic solvent.
  • the reaction at right in contrast, is carried out in the presence of an organic solvent, and with removal of the fatty acid.
  • the fatty acid is hydrolyzed from the lactosylceramide by treatment with a base and water (Step 1).
  • a sialic acid residue is then added by enzymatic transfer to the galactose residue using an ⁇ 2,3 sialyltransferase, preferably an ST3GalIV (Step 2).
  • This reaction can be carried out in the presence of an organic solvent.
  • a GaNAc residue is then attached to the galactose in a ⁇ 1,4 linkage using a GalNAc transferase (Step 3); this step may or may not be carried out in the presence of an organic solvent.
  • the fatty acid moiety is reattached to the sphingosine to obtain the desired GM 2 ganglioside.
  • the reaction typically proceeds nearly to completion due to the presence of an organic solvent during the sialylation.
  • This Example describes three alternative procedures for the synthesis of the GM2 ganglioside using plant glucosylceramide as the precursor (FIG. 3 ).
  • ⁇ 1,4-galactosidase is used to catalyze the transfer of a Gal residue to the glycosylceramide.
  • an ⁇ 2,3-sialyltranasferase is used in a sialyltransferase cycle to link a sialic acid residue to the Gal.
  • a ⁇ 1,4-GalNAc transferase is added to the reaction mixture, either with UDP-GalNAc or as part of a GalNAc transferase cycle.
  • the GalNAc residue is linked to the Gal residue in an ⁇ 2,3 linkage.
  • Route 2 differs from the synthesis shown in Route 1 in that the addition of the Gal to the glycosylceramide is catalyzed by a ⁇ 1,4-galactosyltransferase enzyme, using either a galactosyltransferase cycle or UDP-Glc/Gal as the acceptor sugar. Sialylation and addition of GaINAc are carried out as described above to obtain GM2.
  • the fatty acid is first removed by treatment with aqueous base prior to the glycosyltransferase steps.
  • the galactosylation, sialylation, and GalNAc transferase reactions are carried out as in Route 2.
  • a fatty acid is linked to the molecule.
  • the fatty acid can be the same as that originally found on the plant glucosylceramide, or can be different. In the example shown in FIG. 3, an activated C 18 fatty acid is used, resulting in the synthesis of GM2. Greater efficiency is generally observed when the fatty acid is removed prior to the glycosylation reactions.
  • This Example describes three alternative procedures for the synthesis of the GM2 and other gangliosides using a glucosylceramide as the precursor (FIG. 4 ).
  • a ⁇ 1,4-galactosidase is used to catalyze the transfer of a Gal residue to the glycosylceramide.
  • an ⁇ 2,3-sialyltranasferase is used in a sialyltransferase cycle to link a sialic acid residue to the Gal.
  • a ⁇ 1,4-GalNAc transferase is added to the reaction mixture, either with UDP-GalNAc or as part of a GalNAc transferase cycle.
  • the GalNAc residue is linked to the Gal residue in an ⁇ 2,3 linkage.
  • Route 2 differs from the synthesis shown in Route 1 in that the addition of the Gal to the glycosylceramide is catalyzed by a ⁇ 1,4-galactosyltransferase enzyme, using either a galactosyltransferase cycle or UDP-Glc/Gal as the acceptor sugar. Sialylation and addition of GalNAc are carried out as described above to obtain GM2.
  • the fatty acid is first removed by treatment with aqueous base prior to the glycosyltransferase steps.
  • the galactosylation, sialylation, and GalNAc transferase reactions are carried out as in Route 2.
  • a fatty acid is linked to the molecule.
  • an activated C 18 fatty acid is used, resulting in the synthesis of GM2. Greater efficiency is generally observed when the fatty acid is removed prior to the glycosylation reactions.
  • additional glycosyltransferases can be used to add additional saccharide residues in order to obtain more complex gangliosides.

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